1. Field of the Invention
The present invention relates to an antenna device, and more particularly to an antenna device capable of controlling its directivity, and a radio communication apparatus using the antenna device.
2. Description of the Related Art
Most mobile communication terminals use a nondirectional antenna as an antenna device, because the direction toward a base station to be communicated with always changes. Meanwhile, in recent years, cellular phones, as an example of mobile communication terminals, have come to have a large number of functions. Those functions include website browsing, video telephone, photographing still images or moving images, navigation using the global positioning system, and authentication and checkout using a radio frequency identification technique. In order to implement such various functions, excellent antenna characteristics must be maintained irrespective of the use states of the cellular phones.
Known as antennas for improving antenna characteristics are diversity antennas, array antennas, Yagi-Uda antennas, patch antennas, and the like. In particular, as disclosed in JP 2000-312112A, a single patch antenna can form a desired radiation area. The patch antenna has a first electricity feeding point and a second electricity feeding point on an X-axis and a Y-axis, respectively, the X-axis and the Y-axis being orthogonal to each other on a conductor patch. Electrical signals fed to these electricity feeding points are different in at least one of amplitude and phase. The patch antenna resonates in directions parallel to the X-axis and also parallel to the Y-axis. As a result, the radio wave of a radio signal, which includes two types of linear polarization orthogonal to each other having the same resonance frequency, is radiated in a Z-axis direction which is opposite to a grounded conductor.
In the patch antenna, vertical polarization and horizontal polarization are always orthogonal to each other, and the direction of the directivity is always in the vertical direction (z-axis direction). Accordingly, in the patch antenna, only the polarization planes change, and the direction of the directivity does not change. In the case of patch antenna, plural patch antennas are arranged in an array in order to incline the direction of the directivity from the vertical direction to the horizontal direction. It is difficult to change the direction of the directivity with the use of only a single patch antenna.
On the other hand, a compact patch antenna having one electricity feeding point is disclosed in the following document: “Compact WLAN Disc Antennas” written by Neil J. McEwan, Raed A. Abd-Alhameed, Embarak M. Ibrahim, Peter S. Excell, and Nazar T. Ali, IEEE transactions on antennas and propagation, vol. 50, No. 12, December 2002. It is also difficult for this antenna to change the directivity.